1. Field of the Invention
This invention relates to the production of aluminum metal from aluminous raw materials via aluminum chloride hexahydrate (ACH) and in particular to an improved process for treating ACH prior to its reductive chlorination to anhydrous aluminum chloride suitable for electrolytic reduction to aluminum.
2. Description of the Prior Art
The Bayer-Hall-Heroult process has been used for the reduction of bauxitic ore to aluminum metal for about 90 years. It is the only commercial process for producing aluminum metal and is very energy intensive. The overall process from the mining of ore to the electrolytic reduction of alumina to aluminum metal consumes about 200.times.10.sup.6 BTU/ton of aluminum which is only about 10% energy efficient.
Bauxite ore which predominantly occurs within the equatorial regions, is mined, dried and finely ground before being fed to the Bayer step. The Bayer step, which is applicable to bauxitic ores only, involves a hot, high-pressure caustic (NaOH) leach which dissolves the aluminum content as sodium aluminate (NaAlO.sub.2). Other impurities such as hematite (Fe.sub.2 O.sub.3), titania (TiO.sub.2) and silica (SiO.sub.2) are insoluble and are separated from the pregnant leach liquor by thickening and filtration. Alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O) is precipitated from the sodium aluminate by seeding with trihydrate crystals and by cooling and dilution. The precipitated trihydrate is washed to remove most of the caustic and then calcined to over 1100.degree. C. to produce Al.sub.2 O.sub.3 low in hydrogen as a feed to the aluminum smelter. The alumina is dissolved in molten cryolite at about 960.degree. C. and electrolyzed with carbon anodes to produce aluminum metal and primarily CO.sub.2 off gas. The electrolysis step requires, typically, about 7.5 kwh/lb (78,750 BTU/lb) with the most modern cells operating at about 6 kwh/lb (63,000 BTU/lb). If the most modern, energy efficient mining, drying, and Bayer processing is combined with the best smelting operation, the total energy consumption would be about 175 MBTU/ton. Since more than two thirds of the energy consumption is in the electrolytic step, efforts have been directed to reducing this energy intensive step. It is known that anhydrous aluminum chloride can be electrolytically reduced at low temperatures of about 700.degree. to 750.degree. C. to produce aluminum and chloride at less than 4 kwh/lb (42,000 BTU/lb) or a reduction of 35 to 45 percent compared to Hall technology. However, the energy to produce anhydrous aluminum chloride must be subtracted from the energy savings to achieve the net energy savings.
Many processes have been proposed to economically produce anhydrous aluminum chloride in order to take advantage of the energy savings that can be accomplished through the electrolysis of AlCl.sub.3. A prerequisite of the AlCl.sub.3 feed is that it must be essentially free of moisture. This results in the requirement that production of the very moisture sensitive AlCl.sub.3 be on site with the electrolytic cells to prevent moisture pick-up in transit.
The processes proposed to produce anhydrous AlCl.sub.3 include the direct chlorination of ores, fly ash and purified Bayer alumina. In the chlorination of ores or fly ash, the metallic impurities such as iron, titanium, silicon, sodium, and the like are also chlorinated and must be separated from the AlCl.sub.3 for it to be a satisfactory feed to the electrolytic cell. Unfortunately, satisfactory separation processes are not available. If it were possible to separate the chloride components, the impurity chlorides would require additional processing to recover the valuable chlorine component for recycle chlorination. Additionally, if it were possible to directly chlorinate an ore, it would be necessary to locate the smelter at the mine since the anhydrous AlCl.sub.3 cannot be successfully transported. Usually, favorable electrical rates required for smelters are not available at mine sites. Consequently, the most successful chlorination process known utilizes the purified product Bayer alumina, the same feed as used in the Hall cell.
Bayer alumina typically contains about 0.5% sodium as Na.sub.2 O that arises from the alkali liquor when the sodium aluminate is precipitated as trihydrate. The sodium content does not adversely affect the Hall cell electrolysis since the electrolytic bath contains sodium as NaF in cryolite. However, when Bayer alumina is reductively chlorinated, the sodium also chlorinates forming NaAlCl.sub.4, which has a melting point of approximately 150.degree. C. This low melting compound results in a liquid in an otherwise gaseous system and causes extensive corrosive damage at the typical alumina chlorination temperatures of 700.degree. to 1000.degree. C. Also, valuable chlorine is used. Thus, in any economically viable process the NaAlCl.sub.4 must be separated and oxidized to release the chlorine for recycle, followed by disposal of NaAlO.sub.2.
The typically high calcination temperatures used (1000.degree. to 1280.degree. C.) to produce Bayer alumina for Hall cell feed result in particles with comparatively low activity with respect to reductive chlorination. Consequently, high chlorination temperatures are required to achieve reasonable chlorination rates. The chlorination temperature for Bayer alumina with solid reductant will be above 700.degree. C. and typically is 800.degree. to 1000.degree. C. (A. Landsberg, "Some Factors Affecting the Chlorination of Kaolinic Clay", Met. Trans. B. AIME 8B, September 1977, p. 435). Thus, a high degree of capital sensitivity results due to the need for high temperatures and the corrosion problem related to the NaAlCl.sub.4 present. Also, more carbon is required in the reaction because at high temperatures more CO is produced. For example, at or below 700.degree. C., the molar carbon to aluminum ratio approaches 0.75 and typically corresponds to 0.33 lb C/lb Al, whereas at 900.degree. C. the molar ratio of carbon to aluminum approaches 1.5 and typically corresponds to carbon consumption of 0.67 lb C/lb Al (Alder et al., "The Chlorination of Alumina: A Comparison of the Kinetics With Different Reduction Agents", Light Metal, 1979, p. 337). The reaction rate at 700.degree. C. is too slow to be practical and at 900.degree. C. and above the carbon cost is prohibitive.
U.S. Pat. No. 4,264,569 teaches a process for producing anhydrous aluminum chloride for use in electrolytic cells wherein the ACH is heated at a temperature range of 200.degree.-450.degree. C. until it is substantially dehydrated and thereafter reacting the dehydrated material in the presence of a gas mixture of chlorine, carbon monoxide, carbon dioxide and hydrogen to produce gaseous anhydrous aluminum chloride. Even though this approach overcomes many of the problems associated with the high temperature chlorination of alumina, it was not recognized that calcining ACH within the temperature range specified in the prior art has the disadvantage of producing a material with a high concentration of residual hydrogen. This residual hydrogen in the ACH results in the unproductive consumption of valuable chlorine and makes the process of producing anhydrous aluminum chloride uneconomical.